A heat-radiation type flow sensor is used as an airflow sensor as disclosed in U.S. Pat. No. 3,747,577 (JP-B2-49-48893). This airflow sensor includes a first current path comprising a heater resistor J1 and a resistor J2 connected in series to the heater resistor J1 as well as a second current path comprising a temperature detection resistor J3 and a resistor J4 connected in series to the temperature detection resistor J3. In practical use, the first and second current paths are connected in parallel to each other between an NPN transistor J6 and the ground. An electrical potential appearing at a junction between the heater resistor J1 and the resistor J2 is supplied to a non-inverting input terminal (+) of an operational amplifier J5. On the other hand, an electrical potential appearing at a junction between the temperature detection resistor J3 and the resistor J4 is supplied to an inverting input terminal (−) of the operational amplifier J5.
An electrical potential appearing at the output terminal of the operational amplifier J5 is supplied as a base voltage to the base of the NPN transistor J6 for controlling the flow of a current from a power supply Vb to the first and second current paths. The electrical potential appearing at the junction between the temperature detection resistor J3 and the resistor J4 is taken as the output electrical potential V0 of the airflow sensor. This output electrical potential V0 of the airflow sensor is supplied to a control circuit J7, so that it may be used as a value of an airflow detection result.
The electrical potential appearing at the output terminal of the operational amplifier J5, that is, the voltage supplied to the base of the NPN transistor J6 changes on the basis of a difference in electrical potential between the input terminals of the operational amplifier J5. Thus, the magnitude of the current flowing to the first and second current paths is controlled in accordance with the difference in electrical potential between the input terminals of the operational amplifier J5.
In addition, in accordance with the flow rate of a fluid flowing over the heater resistor J1, the resistance of the heater resistor J1 changes and the way the resistance of the temperature detection resistor J3 placed in the vicinity of the heater resistor J1 changes also varies as well. Thus, the electrical potentials appearing at the input terminals of the operational amplifier J5 also vary in accordance with the flow rate of the fluid flowing over the heater resistor J1. As a result, the magnitude of the current flowing to the first and second current paths is controlled in accordance with the flow rate of the fluid flowing over the heater resistor J1.
In recent years, it is required that the heat-radiation type flow sensor is designed to better withstand electromagnetic interferences (EMI). The conventional circuit configuration has a positive feedback circuit inputting the electrical potential appearing at a junction between the heater resistor J1 and the resistor J2 to the non-inverting input terminal of the operational amplifier J5 as well as a negative feedback circuit inputting the electrical potential appearing at a junction between the temperature detection resistor J3 and the resistor J4 to the inverting input terminal of the operational amplifier J5.
Since a circuit system including the positive feedback circuit is instable, the circuit system is not capable of well enduring electromagnetic interferences. It is thus likely that the circuit system oscillates. Thus, a countermeasure for coping with electromagnetic interferences is needed. An example of such a countermeasure is the use of an EMI filter. However, the EMI filter adds costs.